Divalent Barium Research Articles

Optical study of the metal-insulator transition on Ba1–xKxBi03 thin films

THE superconducting perovskite (Ba, K)BiO3 (refs 1, 2) has the highest transition temperature known (Tc≈css31 K) aside from the copper oxide compounds3. This system becomes superconducting upon the substitution of monovalent potassium for divalent barium in the parent insulator BaBiO3, analogous to the substitution of strontium for lanthanum in La2CuO4. To understand whether the mechanism of the superconductivity is the same as in the copper oxide compounds, it is important to know the origin of the insulating state of the parent material, as well as the basic character of the metallic state induced by the cation substitution. In the case of its predecessor Ba(Bi,Pb)O3, optical measurements were used to investigate the change in electronic state across the metal-insulator transition at 65% lead doping4. Here we report the successful synthesis of (Ba,K)BiO3 thin films, which has allowed us to measure the optical spectrum, as well as the transport coefficients of (Ba,K)BiO3. Our results, combined with the previous report of the X-ray crystal structure5, strongly suggest that the Peierls gap, responsible for the insulating state of BaBiO3, almost completely vanishes at the metal-insulator transition, resulting in a metallic state described by band theory6.

Complex Formation and Ionic Association in Methanol Solutions of Poly(Ethylene Oxide) and Alkaline Earth Salts

Abstract Complex formation of poly(ethylene oxide) (PEO) with divalent barium and strontium salts was investigated in methanol. In these systems the complexation was accompanied by a considerable degree of ionic association. An analytical model for the polymer-ion complexation based on a one-dimensional lattice model was proposed. According to this model, the electrostatic effects between the bound ions were separated from the total free energy change of the binding. Three binding constants, i.e., the ionic association constant K A, the cation binding constant, K c, and the anion binding constant, K a, could be estimated. K A for barium and strontium salts was comparable, and the effect of counteranions on K A was not large. K c for barium salts was almost independent of the kind of counteranion and larger than that for corresponding strontium salts, indicating stronger polymer-ion interaction for barium salts. The anion binding constant, K a, was strongly dependent upon the kind of anion, and the order w...

Ion interactions in (1-13C)D-Val8 and D-Leu14 analogs of gramicidin A, the helix sense of the channel and location of ion binding sites.

Ion-induced chemical shifts in the carbonyl carbon resonances of synthesized ad verified (1-13C)D-Val8 gramicidin A and (1-13C)D-Leu14 gramicidin A are utilized in combination with the previously determined location of the ion binding sites of the gramicidin A channel (using the carbonyls of L-residues) to determine that the helix sense of the gramicidin A channel) is left-handed. Having resolved the handedness issue, the location of the ion binding sites (which are fundamental to understanding the mechanism of ion transport) are further delineated with the results indicating two sites separated by just over 20 A. Furthermore, the demonstration that the divalent barium ion interacts at the binding site while not being transported through the channel is used to argue that the mechanism of monovalent vs. divalent cation selectivity is due to the positive image force contribution to the central barrier.

Structure refinement of a barium mica

Abstract The crystal structure of 1M-BaLiMg2AlSi3O10F2 has been determined from single-crystal diffractometer data and refined to an R value of 7.1% by least-squares methods. The synthetic barium mica belongs to the monoclinic space group C2/m with a = 5.2858(2), b = 9.1575(6), and c = 10.0375(5) Å, β = 100.124(4)° and Z = 2. There is no Li,Mg ordering in the octahedral sites or Al,Si ordering in the tetrahedral sites. The average bond lengths are (Al,Si)–O = 1.645, (Mg,Li)–O(F) = 2.066, inner Ba–O = 2.975, outer Ba–O = 3.192 Å. Compared to other micas, the large divalent barium ion reduces the interlayer separation and restricts rotation within the tetrahedral layer.

Energy band structure of Ca, Sr and Ba

Attempts have been made to apply the `quantum defect method', in conjunction with the `composite wave variational method', to compute the band structure of divalent metals calcium, strontium and barium. The quantum defect data have been deduced from experimental spectroscopic eigenvalues of singly-ionized Ca, Sr and Ba. The presence of an extra electron in each atomic cell has been taken into account by assuming a uniform charge density which produces a potential -2Z/r - Zr2/r3s, rs being the radius of the Wigner-Seitz cell and Z being the valency of the metal. The calculation of the band has been performed for all these metals neglecting - Zr2/r3s. Then an attempt has been made to take into account the term - Zr2/r3s in the case of Ca. Both these calculations show that the Fermi surface of Ca is dismembered around the symmetry point W inside the Brillouin zone.

A New Catalyst System for Cross-Linking Cotton Cellulose with Formaldehyde

A dual catalyst system containing magnesium and fluoborate ions facilitates the cross-linking of cotton cellulose with formaldehyde. Compared with previous formaldehyde cross-linking processes, the dual catalyst system permits operating at lower temperatures, shorter curing times, and lower concentrations of formaldehyde to produce high and nonvariable wash-wear characteristics which are durable in home and commercial launderings. In addition, this dual catalyst system may be used to impart durable-press properties to cotton fabric by using either pre-cure, partial cure, or post- cure techniques. The magnesium cation seems to be unique in the dual catalyst system, since other divalent cations, e.g., barium and zinc, do not act synergistically with fluoborate. The role of fluoborate is discussed and data are shown which indicate that boron trifluoride is the active species which accelerates the cross-linking reaction in the presence of magnesium. 14Carbon tracer studies reveal that, under equivalent processing conditions, more formaldehyde is bound to cellulose by the dual catalyst system than by a single catalyst (magnesium chloride) system.